The present invention relates to an apparatus for forming minute holes in a workpiece through an electrical discharge machining (EDM) or electroerosive machining process.
Electrical discharge machining (EDM) has conventionally been deemed inappropriate for forming minute holes in a workpiece for the reasons that machining accuracies such as surface roughness and out-of-roundness become more and more problematic as the diameter of a hole formed is smaller, and it is difficult to feed the electrode in minute steps.
Conventional EDM machines employ a ball-and-roller bearing by which the electrode is rotatably supported. The ball-and-roller bearing however cannot avoid displacements of the electrode by a few micrometers. The electrode assembly comprises a spindle construction as a whole which has a large stray capacity and hence cannot reduce the capacitance of an RC circuit. Where holes having a diameter of about 50 micrometers are formed by such an electrode assembly, accuracies such as out-of-roundness and surface roughness become extremely poor. The electrode assembly is heavy and its speed of response for feeding the electrode is necessarily reduced. Therefore, the electrode assembly has no smooth machining capability, and can machine holes in a practical range of diameters down to about 100 micrometers at minimum.
Another problem with the prior EDM machines is that it is quite a complex task to position the electrode accurately for forming a minute hole. Two processes have been available for positioning the machining elecrode and workpiece with respect to each other in forming a hole having a diameter of 0.3 mm or smaller.
One positioning process is known as a contact sensing process in which an electric short circuit is sensed between the machining electrode and the workpiece, a contact position is read on a position scale or the like, and the electrode and the workpiece are relatively moved to a desired position according to the reading on the position scale. According to this process, a weak voltage is applied between a machining electrode and a workpiece placed in an insulative liquid contained in a machining bath on an X-Y table, while at the same time the machining electrode is moved horizontally toward the workpiece. When the machining electrode contacts the workpiece, a shortcircuiting current flowing between the machining electrode and the workpiece is detected and the reading on the position scale at this time is set as 0. Assuming that a desired position where a hole is to be pierced is spaced a distance a in the direction of X from the position in which the workpiece and the machining electrode are in contact with each other, the X-Y table is moved a distance a+d/2 in the direction of X taking the diameter d of the machining electrode into account, so that the central axis of the machining electrode will be in registration with the desired position. When the X-Y table is moved another distance b+d/2 in the direction of Y, the center of the machining electrode is brought accurately into registration with another position in which to pierce another hole.
The above process can provide a sufficient degree of accuracy where the machining electrode has a relatively large diameter. However, in case the machining electrode has a diameter of a few tens micrometers for forming minute holes, the electrode lacks desired rigidity and tends to flex upon contact with the workpiece, resulting in the failure of accurate positioning.
Another positioning process uses a stereomicroscope for observing the tip of the machining electrode and the workpiece while positioning them relatively to each other. This positioning process however also fails to effect accurate positioning because of the parallax since the workpiece is obliquely observed.